The present invention relates to an apparatus and method utilizing a calculator and register means for monitoring calibration for quality control of hematology sample analysis.
The basic hematology measurement, means are provided for measuring such hematological parameters as red blood cell count, white blood cell count and hemoglobin or diluted blood samples. Further parameters which may be measured include mean corpuscular volume and hematocrit. Additional useful parameters which may be calculated either automatically or based on other readings are mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC). Each parameter is of diagnostic significance. Commonly known electronic instruments for measuring parameters must be initially calibrated, and calibration must be monitored to assure that electronic signals indicative of measured parameters are within allowable limits of precision. Instrumentation results, namely measured values for given inputs, can vary in the course of operation.
Commonly, calibration on a hematology parameter measuring instrument is maintained by calibrating the instrument to a standard and periodically running control samples after the testing of predetermined numbers of patient samples. This commonly applied approach, while reliable and satisfactory, is expensive. Further, the assay sheet accompanying a control reagent tells the operator in advance what to expect. The running of controls requires the operational effort of testing additional samples as well. Occasionally, instability of standards or controls can lead to a false indication of loss of calibration.
In order to monitor calibration of a hematology measurement instrument at frequent periodic intervals without the use of control samples, a prior art method referred to herein as the "moving average method" has been developed. This method is known in the art and is explained, for example, in B. S. Bull et al, "A Study of Various Estimators for the Derivation of Quality Control Procedures from Patient Erythrocyte Indices," American Journal of Clinical Pathology, Vol. 61, No. 4, April 1974, pp. 473-481. It has been found that of all the parameter values obtained for hematology samples, both MCV and MCHC are parameters to whose values it is preferred to apply estimator functions for monitoring whether values obtained are within calibration limits. The estimator functions may be applied to values obtained for groups of samples even though the samples are drawn from hospital patients who are in all probability hematologically sick. One such estimator is called by Bull et al X.sub.B and named the alternative moving average. EQU X.sub.B,i =(2-r)X.sub.B,i-1 +rd
where X.sub.B,i is the estimator of the true mean after i batches of patient samples have been measured and X.sub.B,i-1 is the estimator after (i-1) batches, r is greater than zero and less than or equal to 1, and d is some sign function equal to plus or minus one or equal to zero, depending on whether or not a particular value X.sub.j within batch i is greater than, equal to, or less than the value X.sub.B,i-1. The function ##EQU1## where P=1/2 and N is the number of samples in the batch. This value may be calculated on a programmable calculator. Use of this function for the average value of a parameter MCV or MCHC should transform raw laboratory data indicative of the values of averages within successive batches into a relatively flat curve. If the curve departs by a preselected amount from a baseline value for the curve, then the condition of the instrument being out of calibration is indicated, and the operator may adjust calibration of the instrument. It should be noted that in the present context, the term "moving average method" is used to mean a method in which an estimator function is applied to measurement of means for parameters of specific batches, and need not be the specific estimator method described above.
While the moving average method for monitoring calibration is a useful tool, its use is primarily directed toward monitoring loss of calibration due to slowly occurring phenomena such as electronic drift. However, step changes, i.e. abrupt changes, in calibration may result as a result, for example, of mechanical or electronic breakdown or clogging of fluid lines. The moving average method is specifically designed not to provide a rapid response to a step change calculating the value of the estimator employs a "smoothing" function.
It is desirable to provide a method and apparatus for hematology parameter measuring apparatus calibration monitoring which is both cost effective and responsive to long and short term changes in system calibration.